1. Field of the Invention
This invention relates to a water-absorbent resin composition which is a mixture of a plurality of water-absorbent resins differing in form and obtained by different methods of polymerization and which excels in absorption characteristics such as an absorption capacity, permeability (flow conductivity of physiological saline), a capillary absorption capacity, the first transition of initial absorption, and the decrease in an amount of rewet of absorbed fluid.
2. Description of the Related Art
Water-absorbent resins capable of absorbing water to tens to hundreds of times their own weights have been developed and have been used for absorption and retention of water such as, first of all, in the field of sanitary materials including sanitary goods and disposable diapers, the field of agricultural and horticulture materials, the field of foodstuffs demanding preservation of freshness, and the industrial fields specializing in prevention of dew formation and heat insulation. As such water-absorbent resins, the hydrolyzate of starch-acrylonitrile graft copolymer, the product of neutralization of starch-acrylic acid graft polymer, the product of saponification of vinyl acetate-acrylic ester copolymer, the hydrolyzate of acrylonitrile copolymer or the product of cross-linkage thereof, cross-linked polymers such as polyethylene imine cross-linked polymer and polyallyl amine cross-linked polymer, and the cross-linked polymer of partially neutralized polyacrylic acid salt have been known.
The water-absorbent resin is requested the quality thereof depending on the purpose for which it is used. The characteristic quality which the water-absorbent resin is intended for such a sanitary material as disposable diaper, for example, is required to manifest resides in attaining a high absorption capacity with an aqueous liquid under a high pressure. For this purpose is available a technique which comprises causing molecular chains in the proximity of the surface of the water-absorbent resin to cross-link thereby increasing the crosslink density of the surface layer, securing among the particles such voids as allow the liquid to migrate when the polymer absorbs water and swells, thereby preventing the absorption capacity of the water-absorbent resin from degrading even when the resin is exposed to high pressure. Since the size of a surface area generally contradicts the absorption capacity under pressure, an increase of the surface area of particles of the polymer results in rendering uniform surface cross-linkage of the polymer proportionately difficult because it renders uniform incorporation of a cross-linking agent in the polymer proportionately difficult. To cope with these difficulties, methods of cross-linking treatment to the neighborhood of surface of a surface porous sodium acrylate polymer to a cross-linking treatment (the official gazette of International Publication No. 94/20543, the official gazette of International Unexamined Patent Publication No. 06-505037, and the specification of European Patent No. 0695762) are conceivable. Even in these methods, when the surface cross-link density is heightened to increase the absorption capacity under pressure during the cross-linking treatment of the neighborhood of surface of the porous polymer, the hydrophilicity of the surface is degraded because it is difficult for the porous polymer to control of the cross-link density. In spite of the use of the porous polymer, therefore, such problems as lowering the speed of water absorption have persisted.
A technique for mixing a plurality of water-absorbent resins differing inproperties thereby improving properties of a water-absorbent resin. For example, a water absorbing agent having peaks of particle-size distributions in 1000–500 μm and 750–250 μm has been disclosed (the official gazette of JP-A-11-28355). Since the mixture of coarse particles and fine particles are endowed a proper discontinuous arrangement by gaps of particles having suitable particle-size interval, it said that an absorbent article which satisfied both quick diffusion in the lateral direction and quick absorption in the vertical direction was able to be obtained. A water-absorbent resin in a powdery state, possesses a normal standard deviation of not less than 130 in particle-size distribution, and preferably forms not less than two peaks in the particle-size distribution (official gazette of JP-A-11-246625). For the purpose of obtaining a powdery water-absorbent resin having two or more mutually different average particle sizes, this invention contemplates classifying a water-absorbent resin and then mixing not less than two kinds of water-absorbent resins obtained by the classification and possessing no mutually overlapping ranges of particle size. Also, a water absorbing agent formed by fixing on the surface of water-absorbent resin particles fine water absorptive particles having a lower absorption capacity than the absorption capacity of the water-absorbent resin particles is disclosed (official gazette of JP-A-11-333292). Further, a water-absorbing agent which is obtained by mixing a water-absorbent resin (A1) capable of absorbing physiological saline in a volume of not less than 55 g/g under no load and absorption speed of not less than 40 sec a water-absorbent resin (A2) capable of absorbing physiological saline in a volume of not less than 20 g/g under a load of 40 g/cm2 and possessing a coefficient of gel elasticity of not less than 750 N/m2 at a mixing ratio of (A1):(A2) in the range of (3:7)–(7:3) by weight and which is preferable for absorbing articles is disclosed (official gazette of JP-A-2000-15093). Incidentally, the official gazettes of JP-A-11-28355, JP-A-11-246625, JP-A-11-333292, and JP-A-2000-15093 demonstrate in working examples mixed use of water-absorbent resins obtained by aqueous solution polymerization.
A granular water-absorbent resin characterized produced by binding beads of water-absorbent resin having not less than two frequency distributions and having a ratio of smaller median particle diameters to the largest median particle diameter in the range of 1/3000–1/1.5 and having an average particle diameter in the range of 200–10000 μm is disclosed (official gazette of JP-A-11-130968). The granular water-absorbent resin is obtained by varying the speed of agitation halfway in the course of the reversed phase suspension polymerization.
An absorbent article using a water-absorbent resin of a fast absorption speed and a water-absorbent resin of a slow absorption speed at a ratio in the range of 90/10–10/90 and having an absorbent resin concentration of not less than 80 wt. % is disclosed (official gazette of JP-A-2000-354760). The exaltation of the ability of capillary absorption by combining two super-absorptive powders is also disclosed (official gazette of International Unexamined Patent Publication No. 2000-513392).
The water-absorbent resins in the inventions of the official gazettes of JP-A-11-28355, JP-A-11-246625, JP-A-11-333292, and JP-A-2000-15093 are irregular-shape polymers obtained by crushing polymers polymerized in an aqueous solution. They, therefore, have comparatively low bulk densities and prove unfavorable for the purpose of enabling them to form an absorbing article in a small thickness or allowing them to transport in a compact size. The water-absorbent resins which are used in the invention of the official gazette of JP-A-11-130968 and the official gazette of International Unexamined Patent Publication No. 2000-513392 are obtained by the reverse phase suspension polymerization and are in the form of spheres or an aggregate thereof and, therefore, entail the problem that the water-absorbent resin particles will readily fall down from the absorbent articles during the formation thereof since a bulk specific gravity thereof is relatively high as 0.9 g/ml. The water-absorbent resin obtained by the reverse phase suspension or emulsion polymerization entails the problem of suffering the surface tension of an absorbing liquid such as urine to lower owing to the use of a surfactant or an emulsifier during the production of the resin and consequently suffering the resin, when used as an absorbent article, to increase the amount of rewet of the solution once absorbed, though the mechanism of the rewet(wet back) remains yet to be elucidated in detail.
The term “ability of capillary absorption” mentioned in the official gazette of International Unexamined Patent Publication No. 2000-513392 refers to absorption capacity under pressure and differs completely from capillary suction force in the present invention. As described in detail in the subsequent paragraph, this magnitude is measured by the use of a device illustrated diagrammatically in FIG. 1. Specifically, this measurement is attained by placing a given sample (water-absorbent resin) at a position several tens of cm higher than the level of physiological saline held in a solution vessel and determining the ability of the sample to absorb the saline by capillary against the negative pressure of the water column of the height mentioned above. Heretofore, examples of measuring the ability of absorption of a sample placed in the absence of negative pressure, namely at a position equal to the level of a solution held in a reservoir have been observed (WO88/01282). No example of any water-absorbent resin measured for ability of capillary absorption under such negative pressure has existed up to date. The correlation between the ability of capillary absorption manifested by the water-absorbent resin used in the method of this invention and the mixture of water-absorbent resins has never been known in the art.
The disposable diaper forms one of the practical uses which are found for the water-absorbent resin. Since the baby wearing the disposable diaper moves around incessantly, the load exerted to bear on the water-absorbent resin is not fixed. Even when the water-absorbent resin exhibits a high absorption capacity under a high load, where are times when the ability of the resin to absorb water relative to the pressure is not fixed. Only because the water-absorbent resin exhibits a high absorption capacity under high pressure, namely the load at most in the range of 10—several tens of g/cm2 estimated from the ordinary body weight (in the neighborhood of 10 kg) of a baby, it does not necessarily follow that the resin, when incorporated in an actual disposable diaper, will function fully satisfactorily. The fact that the resin possesses a high ability to absorb water in the absence of the application of pressure and a ability to effect capillary absorption of water under negative pressure and the resin entails only a small amount of rewet are very important factors. When the absorption capacity under pressure is heightened, the absorption capacity under no pressure is lowered because of an increase in the surface crosslink density, with the result that well-balanced absorption properties will not be easily obtained and the absorbent article will not be fully improved in solid state properties.